Actors and actresses use what is called “cognitive dissonance” if they are really good at their trade. What that means is that they literally “become” the person they are portraying. If they are doing a love scene they (at least for the moment and as we all know perhaps in real life) fall in love with the actor playing opposite them.
Writers, if they are really good, use a similar technique which I am sure has some academic term but I don’t know what it is. The writer “becomes” the protagonist and sees and acts through the hero’s or heroine’s eyes.
I was once interviewed for a reason that escapes me now and the interviewer asked me a normal question from non-writers. “How do you come up with the plots and characters?” I have been asked that same question many times and I am sure you have too. Some writers are afraid and reluctant to examine too closely the talent that God has given them, on the fear that looking at it may take it away. I don’t believe that, and I am sure that all authors have different methods of doing their work. I told the interviewer that in my case, writing shows up as if it were a movie playing on the inside of my eyelids. I “see” the scene which I describe and I write the dialog that the characters are saying appropriate to that scene. This gives me the foundation for whatever the plot calls for. After that, it is simply rewriting and rewriting and rewriting until I have “wrung out” everything I can get from that scene.
My fiction is character-driven, but that is just me. I know if I have strong characters who are confident in their “roles” the plot will pretty much take care of itself. I once knew a very popular author quite well and when I would visit him his den had strings drooping across it and he had plot notes clipped to the strings. As he wrote he would rearrange the notes as necessary, but he had the plot down before he ever touched (in those days) a key on the typewriter.
On the other hand there are authors like the late Elmore Leonard who admittedly tossed up a bunch of eccentric characters and waited to see what they would do. And, of course, he was quite successful with this method.
Whatever approach you take it is essential that you know your characters as if they were members of your family. Considering the dysfunctional families out there, perhaps you should know them BETTER than your own family!